Citation or identification of any reference in Section 2 of this application shall not be construed as an admission that such reference is available as prior art to the present invention.
A major problem in the chemotherapy of solid tumor cancers is delivery of therapeutic agents, such as drugs, in sufficient concentrations to eradicate tumor cells while at the same time minimizing damage to normal cells. Thus, studies in many laboratories are directed toward the design of biological delivery systems, such as antibodies, cytokines, and viruses for targeted delivery of drugs, pro-drug converting enzymes, and/or genes into tumor cells. Houghton and Colt, 1993, New Perspectives in Cancer Diagnosis and Management 1: 65-70; de Palazzo, et al., 1992a, Cell. Immunol. 142:338-347; de Palazzo et al., 1992b, Cancer Res. 52: 5713-5719; Weiner, et al., 1993a, J. Immunotherapy 13:110-116; Weiner et al., 1993b, J. Immunol. 151:2877-2886; Adams et al., 1993, Cancer Res. 53:4026-4034; Fanger et al., 1990, FASEB J. 4:2846-2849; Fanger et al., 1991, Immunol. Today 12:51-54; Segal, et al., 1991, Ann N.Y. Acad. Sci. 636:288-294; Segal et al., 1992, Immunobiology 185:390-402; Wunderlich et al., 1992; Intl. J. Clin. Lab. Res. 22:17-20; George et al., 1994, J. Immunol. 152:1802-1811; Huston et al., 1993, Intl. Rev. Immunol. 10:195-217; Stafford et al., 1993, Cancer Res. 53:4026-4034; Haber et al., 1992, Ann. N.Y. Acad. Sci. 667:365-381; Haber, 1992, Ann. N.Y. Acad. Sci. 667: 365-381; Feloner and Rhodes, 1991, Nature 349:351-352; Sarver and Rossi, 1993, AIDS Research & Human Retroviruses 9:483-487; Levine and Friedmann, 1993, Am. J. Dis. Child 147:1167-1176; Friedmann, 1993, Mol. Genetic Med. 3:1-32; Gilboa and Smith, 1994, Trends in Genetics 10:139-144; Saito et al., 1994, Cancer Res. 54:3516-3520; Li et al., 1994, Blood 83:3403-3408; Vieweg et al., 1994, Cancer Res. 54:1760-1765; Lin et al., 1994, Science 265:666-669; Lu et al., 1994, Human Gene Therapy 5:203-208; Gansbacher et al., 1992, Blood 80:2817-2825; Gastl et al., 1992, Cancer Res. 52:6229-6236.
2.1. Bacterial Infections and Cancer
Regarding bacteria and cancer, an historical review reveals a number of clinical observations in which cancers were reported to regress in patients with bacterial infections. Nauts et al., 1953, Acta Medica. Scandinavica 145:1-102, (Suppl. 276) state:                The treatment of cancer by injections of bacterial products is based on the fact that for over two hundred years neoplasms have been observed to regress following acute infections, principally streptococcal. If these cases were not too far advanced and the infections were of sufficient severity or duration, the tumors completely disappeared and the patients remained free from recurrence.Shear, 1950, J. A.M.A. 142:383-390 (Shear), observed that 75 percent of the spontaneous remissions in untreated leukemia in the Children's Hospital in Boston occurred following an acute episode of bacterial infection. Shear questioned:        Are pathogenic and non-pathogenic organisms one of Nature's controls of microscopic foci of malignant disease, and in making progress in the control of infectious diseases, are we removing one of Nature's controls of cancer?        
Subsequent evidence from a number of research laboratories indicated that at least some of the anti-cancer effects are mediated through stimulation of the host immune system, resulting in enhanced immuno-rejection of the cancer cells. For example, release of the lipopolysaccharide (LPS) endotoxin by gram-negative bacteria such as Salmonella triggers release of tumor necrosis factor, TNF, by cells of the host immune system, such as macrophages, Christ et al., 1995, Science 268:80-83. Elevated TNF levels in turn initiate a cascade of cytokine-mediated reactions which culminate in the death of tumor cells. In this regard, Carswell et al., 1975, Proc. Natl. Acad. Sci. USA 72:3666-3669, demonstrated that mice injected with bacillus Calmette-Guerin (BCG) have increased serum levels of TNF and that TNF-positive serum caused necrosis of the sarcoma Meth A and other transplanted tumors in mice. Further, Klimpel et al., 1990, J. Immunol. 145:711-717, showed that fibroblasts infected in vitro with Shigella or Salmonella had increased susceptibility to TNF.
As a result of such observations as described above, immunization of cancer patients with BCG injections is currently utilized in some cancer therapy protocols. See Sosnowski, 1994, Compr. Ther. 20:695-701; Barth and Morton, 1995, Cancer 75 (Suppl. 2):726-734; Friberg, 1993, Med. Oncol. Tumor. Pharmacother. 10:31-36 for reviews of BCG therapy.
2.2. Parasites and Cancer Cells
Although the natural biospecificity and evolutionary adaptability of parasites has been recognized for some time and the use of their specialized systems as models for new therapeutic procedures has been suggested, there are few reports of, or proposals for, the actual use of parasites as vectors.
Lee et al., 1992, Proc. Natl. Acad. Sci. USA 89:1847-1851 (Lee et al.) and Jones et al., 1992, Infect. Immun. 60:2475-2480 (Jones et al.) isolated mutants of Salmonella typhimurium that were able to invade HEp-2 (human epidermoid carcinoma) cells in vitro in significantly greater numbers than the wild type strain. The “hyperinvasive” mutants were isolated under conditions of aerobic growth of the bacteria that normally repress the ability of wild type strains to invade HEp-2 animal cells. However, Lee et al. and Jones et al. did not suggest the use of such mutants as therapeutic vectors, nor did they suggest the isolation of tumor-specific bacteria by selecting for mutants that show infection preference for melanoma or other cancers over normal cells of the body. Without tumor-specificity or other forms of attenuation, such hyperinvasive Salmonella typhimurium as described by Lee et al. and Jones et al. would likely be pan-invasive, causing wide-spread infection in the cancer patient.
2.3. Tumor-Targeted Bacteria
Genetically engineered Salmonella have been demonstrated to be capable of tumor targeting, possess anti-tumor activity and are useful in delivering effector genes such as the herpes simplex thymidine kinase (HSV TK) to solid tumors (Pawelek et al., WO 96/40238). Two significant considerations for the in vivo use of bacteria are their virulence and ability to induce tumor necrosis factor α (TNFα)-mediated septic shock. As TNFα-mediated septic shock is among the primary concerns associated with bacteria, modifications which reduce this form of an immune response would be useful because TNFα levels would not become toxic, and a more effective concentration and/or duration of the therapeutic vector could be used.
2.4. Modified Bacterial Lipid A
Modifications to the lipid composition of tumor-targeted bacteria which alter the immune response as a result of decreased induction of TNFα production were suggested by Pawelek et al. (Pawelek et al., WO 96/40238). Pawelek et al. provided methods for isolation of genes from Rhodobacter responsible for monophosphoryl lipid A (MLA) production. MLA acts as an antagonist to septic shock. Pawelek et al. also suggested the use of genetic modifications in the lipid A biosynthetic pathway, including the mutation firA, which codes for the third enzyme UDP-3-O (R-30 hydroxylmyristoly)-glucosamine N-acyltransferase in lipid A biosynthesis (Kelley et al., 1993, J. Biol. Chem. 268: 19866-19874). Pawelek et al. showed that mutations in the firA gene induce lower levels of TNFα. However, these authors did not suggest enzymes which modify the myristate portion of the lipid A molecule. Furthermore, Pawelek et al. did not suggest that modifications to the lipid content of bacteria would alter their sensitivity to certain agents, such as chelating agents.
In Escherichia coli, the gene msbB (mlt) which is responsible for the terminal myristalization of lipid A has been identified (Engel, et al., 1992 J. Bacteriol. 174:6394-6403; Karow and Georgopoulos 1992 J. Bacteriol. 174: 702-710; Somerville et al., 1996 J. Clin. Invest. 97: 359-365). Genetic disruption of this gene results in a stable non-conditional mutation which lowers TNFα induction (Somerville et al., 1996 J. Clin. Invest. 97: 359-365). These references, however, do not suggest that disruption of the msbB gene in tumor-targeted Salmonella vectors would result in bacteria which are less virulent and more sensitive to chelating agents.
The problems associated with the use of bacteria as gene delivery vectors center on the general ability of bacteria to directly kill normal mammalian cells as well as their ability to overstimulate the immune system via TNFα which can have toxic consequences for the host (Bone, 1992 JAMA 268: 3452-3455; Dinarello et al., 1993 JAMA 269: 1829-1835). In addition to these factors, resistance to antibiotics can severely complicate coping with the presence of bacteria within the human body (Tschape, 1996 D T W Dtsch Tierarztl Wochenschr 1996 103:273-7; Ramos et al., 1996 Enferm Infec. Microbiol. Clin. 14: 345-51).
Hone and Powell, WO97/18837 (“Hone and Powell”), disclose methods to produce gram-negative bacteria having non-pyrogenic Lipid A or LPS. Although Hone and Powell broadly asserts that conditional mutations in a large number of genes including msbB, kdsA, kdsB, kdtA, and htrB, etc. can be introduced into a broad variety of gram-negative bacteria including E. coli, Shigella sp., Salmonella sp., etc., the only mutation exemplified is an htrB mutation introduced into E. coli. Further, although Hone and Powell propose the therapeutic use of non-pyrogenic Salmonella with a mutation in the msbB gene, there is no enabling description of how to accomplish such use. Moreover, Hone and Powell propose using non-pyrogenic bacteria only for vaccine purposes.
The objective of a vaccine vector is significantly different from the presently claimed tumor-targeted vectors. Thus, vaccine vectors have requirements quite different from tumor-targeted vectors. Vaccine vectors are intended to elicit an immune response. A preferred live bacterial vaccine must be immunogenic so that it elicits protective immunity; however, the vaccine must not be capable of excessive growth in vivo which might result in adverse reactions. According to the teachings of Hone and Powell, a suitable bacterial vaccine vector is temperature sensitive having minimal replicative ability at normal physiological ranges of body temperature.
In contrast, preferred tumor-targeted parasitic vectors, such as but not limited to Salmonella, are safely tolerated by the normal tissues of the body such that pathogenesis is limited, yet the vectors target to tumors and freely replicate within them. Thus, vaccine vectors which replicate minimally at normal body temperatures, would not be suitable for use as tumor-targeted vectors.